Field of the Invention
The present disclosure relates to an image forming apparatus, and particularly to a fixing device included in an electrophotographic image forming apparatus.
Description of the Related Art
An electrophotographic image forming apparatus performs, as a printing process, a step of forming a toner image on a photosensitive body according to an input image, a step of primarily transferring the toner image on the photosensitive body to a transfer belt, a step of secondarily transferring the toner image on the transfer belt to a sheet, and a step of fixing the toner image to the sheet by heat using a fixing device.
The fixing device includes a heating rotator and a pressurizing roller. The heating rotator includes a heater. The heating rotator heated with the heater transfers heat to a sheet that passes through a contact portion (hereinafter, also referred to as a fixing nip) between the heating rotator and the pressurizing roller. Therefore, the toner is melted and fixed onto the sheet.
Recently, the fixing device including separation members becomes widespread. The separation members are linearly disposed opposite to the heating rotator, and separate the sheet from the heating rotator such that the sheet passing through the fixing nip is not caught in the heating rotator.
As to the fixing device including the separation member, Japanese Laid-Open Patent Publication No. 2013-57775 discloses a fixing device in which the separation member favorably separates a recording medium from a heating member without damaging the heating member even if a temperature of the surface of the heating member changes. Japanese Laid-Open Patent Publication No. 2014-215355 discloses a fixing device capable of suppressing abrasion of a surface of a heating member. Japanese Laid-Open Patent Publication No. 2012-133201 discloses a fixing device capable of maintaining good separativeness while the rotator is not damaged even if a recording medium comes into contact with the rotator. Japanese Laid-Open Patent Publication No. 2009-258396 discloses a fixing device in which a distance between a sheet conveying member and the separation member is controlled to prevent the abrasion and wear of the sheet conveying member caused by the separation member.
A gap (hereinafter, also referred to as a separation gap) is provided between the heating rotator and the separation member. For the large separation gap, the sheet enters the separation gap to cause a sheet jam. On the other hand, for the small separation gap, when the heating rotator is thermally expanded, the separation member comes into contact with the heating rotator to damage the heating rotator. Accordingly, desirably the separation gap is always kept constant.
The sheet takes heat from the heating rotator when passing through the fixing nip. For this reason, when the sheets having various sizes pass through the fixing nip, temperature unevenness occurs in the surface of the heating rotator to vary a degree of thermal expansion in different positions in the heating rotator. Resultantly, the separation gap varies in different positions in the heating rotator.
In the fixing devices of Japanese Laid-Open Patent Publication Nos. 2013-57775, 2014-215355, 2012-133201, and 2009-258396, the positions of the separation members cannot individually be adjusted. Therefore, when the sheets having various sizes pass through the fixing nip, the separation gap varies in different positions in the heating rotator. In this case, a sheet jam is highly likely to occur. Thus, there is a demand for a technique capable of suppressing the sheet jam even if the sheets having various sizes pass through the fixing nip.